Many of the current treatments of cancer, such as photodynamic therapy (PDT), have an oxidative component in their mechanism of action. Our initial studies with nitric oxide and glutathione indicate that they are able to provide substantial protection of cells from the oxidative insult of PDT. An often-overlooked aspect of nitric oxide is its ability to serve as a chain-breaking antioxidant. We hypothesize that at low concentrations nitric oxide will serve as an antioxidant, blunting the effectiveness of PDT in tumor cells or protecting normal cells from damage; however at higher concentrations it will be a pro-oxidant. PDT introduces lipid hydroperoxides into cell membranes. The antioxidant enzyme, phospholipid-hydroperoxide glutathione peroxidase (PhGPx) requires glutathione as a cofactor and is an important enzyme for the removal of these hydroperoxides in the membrane. We have found that modulation of PhGPx in cells will alter the response of cells to PDT. Glutathione provides the reducing equivalents needed to remove excess peroxide from cells and tissues by the glutathione peroxidase family of enzymes. In addition, the equilibrium system composed of the glutathione disulfide/glutathione couple is thought to be central in governing the redox status of cells and tissue. Thus, we hypothesize that the response of cells to cancer treatment protocols that rely on oxidative mechanisms is determined by the redox state of the cell or tissue and its ability to control redox status. The goals of this proposed research are to determine the effects and relationships of nitric oxide, PhGPx and glutathione on PDT. In addition our proposed studies on the glutathione system are unique, as these studies will be the first effort to make a quantitative estimate of cellular redox potential and relate this to an outcome of an anti-cancer therapy.